
Here, we follow [1]}, [2]}, the minimal detectable event rate is set to \(1 \mathrm {cGpc}^{-3} \mathrm {yr}^{-1}\) . \(f_{\mathrm {unc}}\)  is the unconstraint fraction of PBHs in the dark matter where we take \(10^{-3}\) , \(10^{-4}\) , \(10^{-5}\)  for illustration. The black dashed lines correspond with an event rate of \(1 \mathrm {cGpc}^{-3} \mathrm {yr}^{-1}\) . The shadow regions show the effectively detectable mass ranges at high redshift. One thing we need to pay attention to is the maximum detectable redshift is \(z = 3000\) , which comes from the PBH binary model [3]}, the PBH binaries formed at matter-radiation equality \(z_{\mathrm {eq}} = 3000\) . This mechanism can hardly produce PBHs in radiation-domination epoch at \(z > z_{\mathrm {eq}}\) , which makes it difficult in observing a larger mass range and detecting PBH binaries on SKA.
